1. Field of the Invention
The present invention relates to a hologram producing apparatus for producing three-dimensional information of an object and a hologram displaying apparatus for reading out the three-dimensional information of the object from the hologram producing apparatus so as to display a three-dimensional image of the object.
2. Related Background Art
Holography has been noted as a technique for displaying a three-dimensional image of an object. It includes a hologram producing technique for producing three-dimensional information of an object and a hologram displaying technique for reading out the three-dimensional image recorded by the hologram producing technique so as to display a three-dimensional image of the object.
Conventional holography has been constructed on the premise that its recording device has a high resolution. Accordingly, most of hologram producing apparatuses use high-resolution photographic dry plates or thermoplastics as their recording devices. These devices utilize their characteristically high resolution so as to set a relatively large angle between reference light and object light, thereby producing a hologram based on actual imaging or by recording results of calculations performed by a computer or the like. Holograms thus obtained can be reproduced by the so-called off-axis method, whereby the image quality can be prevented from deteriorating due to conjugate images.
Such a hologram producing technique is basically a high-grade photographic technique due to its high resolution, thereby necessitating an enormous amount of labor for capturing a hologram. Also, since it requires the photographic dry plate or thermoplastic to be developed, operations may not sufficiently be performed in real time.
Thus, techniques of holography using a CCD camera, which is a low resolution relatively low-resolution imaging device, have been proposed in Sato et al., the Journal of the Institute of Television Engineers of Japan, Vol. 45, No. 7, pp. 873-875 (1991), referred to as "conventional example 1" hereinafter, and Hashimoto, the Journal of the Institute of Image Electronics Engineers of Japan, Vol. 22, No. 4, pp. 315-322 (1991), referred to as "conventional example 2" hereinafter, for example.
The conventional example 1 exemplifies a technique of Fresnel type holography in which a lens normally employed in the holographic imaging technique is not used. Also, the holographic imaging technique disclosed in the conventional example 2 uses an imaging lens so as to match the spatial resolution of a real image with that of the imaging device, while disposing an opening directly in front of the imaging lens on the object side, so as to obtain an off-axis type hologram.
Further, a technique of holography in which a computer hologram is reproduced by a low-resolution liquid-crystal spatial light modulator has been proposed in Maeno et al., Three-Dimensional Image Conference '94 Lecture Papers, pp. 165-170, referred to as "conventional example 3" hereinafter. In the conventional example 3, a mask is disposed around a real image to be reproduced, thereby eliminating light from its conjugate image.
Also, as a method for eliminating the conjugate image at the time when a reproduced image is observed, single side band method (O. Bryngdahl et al., J. Opt. Soc. Am., 58, 620, 1968) has been known.
In this method, first, with respect to a two-dimensional planar object whose phase component does not change, a conjugate image is obtained by a confocal optical system, and then a Fresnel hologram of this conjugate image is obtained. In order to obtain this Fresnel hologram, a mask which can transmit therethrough only one side of Fourier components of object light is disposed on a Fourier plane of the confocal optical system, and then the Fresnel hologram of the image constituted by thus transmitted Fourier components of the object light is recorded.
Thereafter, while a confocal optical system in which the wavefront subjected to Fresnel propagation from the hologram by the distance equal to that at the time of recording becomes the object-side focal plane is used, the Fourier components on the side which were not masked at the time of recording are masked, and an image is formed by thus transmitted light, whereby a reproduced image is obtained free of the conjugate image.